Phenol is a very important chemical for the chemical industry due to its widespread use in the fields of resin, plastics, pharmaceuticals, agrochemicals, etc. It is mainly used for the production of a large no of intermediates such as bisphenol, caprolactum, aniline, alkylphenol, chlorophenol, salicylic acid, etc., which are then further used to produce epoxy resin for paints, polycarbonate plastics for CDs and domestic appliances, nylon, polyamides, antioxidants, surfactants, detergents, anticeptics, medicines etc. At present phenol is mainly produced by three steps Cumene Process. However, the process has several disadvantages such as poor ecology, formation of an explosive intermediates (cumene hydroperoxide), multistep character which makes it difficult to achieve high phenol yield w.r.t. benzene. The main concern in the fine chemical and drug intermediates are the amount of waste generated per unit weight of desired product (called E-factor by R A Sheldon in Chemsitry & Industry, 6 Jan. 1997, P 13) and poor atom efficiencies (kg of product produced per Kg of reactants used) due to the use of stoichiometric reagents and minerals acid/base catalysts. In this context, the use of solid catalysts which are eco-safe and reusuable become important. Moreover a major problem with this process is that it produces phenol is driving its price down and also hurting the economics of phenol as well. This concern is the impetus for researchers to develop a direct single step co-product free and environment friendly route to phenol.
There are reports on the production of phenol by direct hydroxylation of benzene with different oxidants over different solid catalyst but to the best of our knowledge there is no reference for the use of molecular oxygen (air) only for this purpose.
Reference may be made to article in the Journal of Physical Chemistry, 1983, 87, 903-905, in which Japanese workers reported the use of nitrous oxide for the hydroxylation of benzene to phenol—using vanadium pentaoxide/silica catalyst at 550° C. to achieve 10% benzene conversion and 70% phenol selectivity.
Reference may also be made to patents WO9527691, 1995 and WO9527560, 1995 wherein Panov et al developed a one step process for the manufacture of phenol from benzene using nitrous oxide as the and ZSM-5 and ZSM-11 as the catalysts. The drawbacks of this process are deactivation of catalyst, loss of selectivity of catalyst and side reaction (combustion of benzene by nitrous oxide). It is economically attractive only if N2O is available as the by product of some other process such as the two step oxidation of cyclohexane to adipic acid.
Reference may be made to article in J. Chem. Soc. Chem. Com., 1992, 1446-1447 wherein Tatsumi et al. describe a process for the preparation of phenol from benzene with H2 and O2 which uses a catalyst consisting of palladium supported on TS-1. Operating according to this process, a conversion of benzene of 0.07% is obtained with a turnover of Palladium of 13.5.
Another reference may be made to European patent EP0894783, 1998, wherein a process for the synthesis of phenol by catalytic oxidation of benzene in the presence of titanium silicate and by H2O2 prepared in situ by reaction of oxygen carbon monoxide and water in the presence of catalytic complexes consisting of palladium with a nitrogenated ligand and a non-coordinating counter ion. The selectivity of benzene to phenol is greater than 95%, but benzene conversions were only 1-2%.
Reference may be made to the article in Journal of Molecular CatalysisA: Chemical 2006, 253, 1-7, wherein phenol is prepared by homogeneous liquid phase direct catalytic oxidation of benzene at room temperature in acetonitrile solvent using sodium metavenadate as the catalyst and hydrogen peroxide as the oxidant. Phenol yield of 13.5% with a selectivity of 94% was reported.
Reference may be made to Ind. Eng. Chem. Res. 1999, 38, 1893-1903, wherein phenol was synthesized by direct liquid phase benzene hydroxylation by H2O2 using V-MCM-41 as the catalyst under mild conditions. Operating accordingly to this process, a conversion of benzene of 13% and selectivity for phenol of 48% was obtained.
Another reference may be made to Science 2002, 105, 295, wherein phenol was obtained by direct vapour phase hydroxylation of benzene using Pd-membrane as a catalyst using O2 and H2 as the oxidant. Phenol yield of 12% and selectivity of 80-97% was obtained.
Another reference may be made to article in Applied Clay Science 2006, 33, 1-6, wherein selective direct hydroxylation of benzene with hydrogenperoxide to phenol was carried out on a clay-supported vanadium oxide catalyst. Under mild reaction conditions at 60° C., high selectivity to phenol of 94% was obtained but conversion of benzene was only 14%.
Another reference may be made to article Angew. Chem. Int. Ed. 2006, 45, 448, wherein phenol was obtained by direct vapour phase hydroxylation of benzene using Re cluster/zeolite as a catalyst using O2 and NH3 as the oxidant. Phenol yield of 5% and selectivity of 80-97% was obtained.
The drawback of the processes reported so far is that they do not exhibit sufficiently high conversions of benzene for high selectivity of phenol to be of interest for industrial application. In most of the cases hazardous oxidizing agent N2O, H2O2 or expensive H2 with O2 or reducing agent NH3 with O2 was used and also lots of unnecessary by-products was formed. In addition, the catalysts used have a limited activity under the operating conditions. There is, therefore, an evident necessity for further improvements in the process for the selective conversion of benzene to phenol.